In the aeronautics field, the regulations in force, such as certifications FAR25.981.b and CS25.981.b, or their equivalents, impose flammability constraints for the fuel contained in aircraft tanks.
In practice, the airframe manufacturer must either be able to prove that the fuel is below these flammability constraints, or establish means for reducing explosion risks.
The use of inerting systems is well known for the generation of an inert gas, such as nitrogen or any other inert gas such as carbon dioxide, and introducing said inert gas into fuel tanks in order to reduce the risk of explosion from said tanks. This solution is widely used for tanks having a large air pocket, i.e., tanks contained in the wings of the aircraft or the central tanks primarily contained in the fuselage of the aircraft.
However, it is difficult to prove compliance with the regulations in force for a fuel manifold whose primary function is to be filled with fuel to be kept constantly full in order to avoid unpriming of the supply pumps of the aircraft engine.
The manifold is intended to communicate with at least one fuel tank. FIG. 1, which schematically shows a manifold (1) according to the prior art, defines an enclosure (2) intended to be filled with fuel and to have an internal pressure P1 higher than the internal pressure P2 of the fuel tank. The enclosure (2) comprises at least one ceiling (3) and at least one side wall (4a) connected to one another so as to define at least one zone (5) between them wherein at least one air pocket (6) can be trapped. The manifold comprises overflow holes (7) arranged in the side wall (4a) near said entrapment zone (5) and intended to communicate with the fuel tank.
According to the state of the art, the overflow holes do not make it possible to discharge the trapped air pocket because structural constraints do not make it possible to position the overflow holes flush with the upper ceiling of the manifold. Furthermore, other air pockets can be trapped, in particular when the filling system of the manifold is not working and/or during particular pitch and/or roll phases.
The manifold is constantly full of fuel, such that the air pocket inside the manifold is relatively small, which limits, or even prevents any injection of inerting gas inside the manifold.
Injecting inerting gas in the manifold also cannot be considered, as it would cause an overpressure of the manifold and the fuel tank in communication therewith, the appearance of bubbles and foam that could harm the operation of the pumps situated in the tank, and an uncontrolled transfer of fuel through the overflow holes.
Moreover, it is that much more difficult to ensure compliance of the manifold with the standard in force, since it is not directly connected to an inert tank, especially if fuel degassing issues are taken into account.
In the current state of the art, to bring the manifold into compliance with the standards in force, a first solution consists in pressurizing the manifold relative to the atmospheric pressure so as to shift the flammability envelope of the fuel. However, pressurization exposes the manifold to greater structural stresses, having a significant impact on the mechanical strength of the aircraft and its operating safety. Furthermore, it is difficult to be precise in estimating this physical phenomenon with respect to the flammability of the fuel.
Another solution consists in injecting inerting gas into the fuel tank that is in communication with the manifold and considering that if the fuel tank meets the requirements of the standards in force, the manifold meets them as well. This solution is based entirely on a qualitative argument, and it is difficult to prove its effectiveness if, as mentioned above, one considers the degassing of the fuel in the manifold. Furthermore, if the fuel tank in communication does not require inerting, this solution does not work.
Another solution consists in coating some inner walls of the manifold with a foam able to prevent the spread of flames in case of ignition of the fuel and to suffocate the fire. However, aside from decreasing the working volume of the manifold, the foam absorbs a quantity of fuel that cannot be retrieved, thereby decreasing the autonomy of the aircraft. Furthermore, the foam can break down and be aspirated by the supply means of the engine. Lastly, static electricity may appear within the foam, giving rise to an ignition risk.